The small papovavirus SV40 induces tumors in experimental animals, and transforms cells in culture. Preliminary experiments have shown that only the early region of the SV40 genome contributes to transformation. Both gene products of the SV40 early region (small t and T-antigen) as well as the viral origin of early transcription, may be required. In the work outlined in this grant, we will isolate and characterize several new classes of SV40 mutants, and use these and existing mutants to identify the products and functions of the early region of SV40 required for transformation and tumorigenesis. The classes of SV40 mutants to be isolated are: 1) small E-mutants, to supplement existing del 54/59 mutants, which may retain partial small t activity. 2) EsA-del 54/59 double mutants. 3) tsA-small E- double mutants. 4) mutants lacking the origin of early transcription. 5) mutants with alterations around the origin of DNA replication. Using rat embryo fibroblasts transformed by wild type SV40 and by these mutants, we will attempt to link the various changes associated with the transformed state (e.g., in growth properties, in plasminogen activator secretion, in metabolic state and in ability to form tumors in nude mice) with the presence of functional small t or T- antigen. We will select from rat embryo fibroblast transformants revertants which have lost specific aspects of the transformed phenotype and attempt to correlate this reversion with specific alterations in the integrated viral genome. Finally, we will evaluate the ability of new and existing SV40 mutants to induce tumors in inbred hamsters, and attempt to correlate this with the phenotype of hamster embryo fibroblasts transformed by the same mutants in vitro.